2 Answers
2

In the near term it's reasonably safe -- the dent is not sufficiently deep to seriously weaken the tube (though one does need to be concerned about the integrity of the welds on the rest of the bike, given it's been in an accident). In the far term (10s of thousands of miles) there's danger that the tube will fatigue and become weakened at the dent. (The thing to watch out for is growing cracks in the paint around the dent, or at frame joints, though some paints will hide cracks more than others.)

As to repair, about the only thing I can think of is to have an experienced frame builder cut out the section and weld in a new one, or fashion some sort of sleeve over the dent. But those solutions would also be subject to long-term fatigue issues.

Repairing and a new paint job would probably as expensive as a new frame (depending on the material of course, but it's Aluminium here, which is not easy to weld).
–
BaarnSep 4 '12 at 13:40

4

If you have the money, though, it'd be safer just to buy a new frame right off rather than wait to see if any welds have been compromised. Aluminum has a tendency to fail suddenly!
–
WTHarperSep 4 '12 at 14:02

Thank you for your answers. I got it to the local shop and they found a crack on the bottom of the tube under the paint. So I will have to change the frame.
–
Peter PopovSep 4 '12 at 14:50

Sorry about the frame! It's in a better place now. If you can, a pic of the crack would be interesting.
–
WTHarperSep 4 '12 at 21:10

Get an empty coke can with no dents or other disfigurements in it, place it upright on the ground, and stand on top of it. It should be able to bear the weight of most adults with normal body mass.

Now put a small dent in the can and try it again. But be careful because the small dent will introduce a weakness into the can and it will almost certainly buckle under your weight this time.

Obviously a bicycle frame is made from much thicker gauge aluminium than a coke can, but the same basic principles hold. A tube is an inherently strong structure, but only if it remains true. You put a dent into it, you put a region into the structure where stresses will concentrate and try to deform the tube further. In the short term, it should be fine, but I suspect the lifespan of the tube would now be measured in months rather than years, as aluminium is a softer metal than steel and is more prone to metal fatigue (alumimium always suffers from stresses that will begin fatigue damage no matter how small the load, according to wikipedia). You also have to consider that there may be other items of less obvious damage, such as small cracks opening up in welds that are small enough to remain hidden by the paint. Fatigue will set in at these points as well.

If you do keep riding on this frame then I'd suggest monitoring the crash damage region very closely. Also, don't go mountain biking on a frame in that state! Stick to the roads. And stick to quiet roads where you're unlikely to hit anything else and ones with no potholes, because another big impact or bump could make the frame fold up like a book at the dent.

As for repair, I suspect the only realistic option is to completely replace that tube. Given how difficult aluminium is to weld I expect the cost of doing that would approach the cost of an entire new frame anyway, and given the frame already has some miles on it and will have accumulated some wear and tear it will not have the life expectancy of a new frame.

I think realistically, your only option is to put that frame out to pasture.